On Wed, Jan 22, 2014 at 10:13:59AM -0800, James Bottomley wrote: > On Wed, 2014-01-22 at 18:02 +0000, Chris Mason wrote: > > On Wed, 2014-01-22 at 09:21 -0800, James Bottomley wrote: > > > On Wed, 2014-01-22 at 17:02 +0000, Chris Mason wrote: > > > > [ I like big sectors and I cannot lie ] > > I think I might be sceptical, but I don't think that's showing in my > concerns ... > > > > > I really think that if we want to make progress on this one, we need > > > > code and someone that owns it. Nick's work was impressive, but it was > > > > mostly there for getting rid of buffer heads. If we have a device that > > > > needs it and someone working to enable that device, we'll go forward > > > > much faster. > > > > > > Do we even need to do that (eliminate buffer heads)? We cope with 4k > > > sector only devices just fine today because the bh mechanisms now > > > operate on top of the page cache and can do the RMW necessary to update > > > a bh in the page cache itself which allows us to do only 4k chunked > > > writes, so we could keep the bh system and just alter the granularity of > > > the page cache. > > > > > > > We're likely to have people mixing 4K drives and <fill in some other > > size here> on the same box. We could just go with the biggest size and > > use the existing bh code for the sub-pagesized blocks, but I really > > hesitate to change VM fundamentals for this. > > If the page cache had a variable granularity per device, that would cope > with this. It's the variable granularity that's the VM problem. > > > From a pure code point of view, it may be less work to change it once in > > the VM. But from an overall system impact point of view, it's a big > > change in how the system behaves just for filesystem metadata. > > Agreed, but only if we don't do RMW in the buffer cache ... which may be > a good reason to keep it. > > > > The other question is if the drive does RMW between 4k and whatever its > > > physical sector size, do we need to do anything to take advantage of > > > it ... as in what would altering the granularity of the page cache buy > > > us? > > > > The real benefit is when and how the reads get scheduled. We're able to > > do a much better job pipelining the reads, controlling our caches and > > reducing write latency by having the reads done up in the OS instead of > > the drive. > > I agree with all of that, but my question is still can we do this by > propagating alignment and chunk size information (i.e. the physical > sector size) like we do today. If the FS knows the optimal I/O patterns > and tries to follow them, the odd cockup won't impact performance > dramatically. The real question is can the FS make use of this layout > information *without* changing the page cache granularity? Only if you > answer me "no" to this do I think we need to worry about changing page > cache granularity. We already do this today. The problem is that we are limited by the page cache assumption that the block device/filesystem never need to manage multiple pages as an atomic unit of change. Hence we can't use the generic infrastructure as it stands to handle block/sector sizes larger than a page size... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
